This invention relates to a magnetic recording medium, and more particularly, to a magnetic recording medium of the metal thin film type.
In the development of magnetic recording media for video, audio, and other applications, recent efforts were made on those having a magnetic layer in the form of a metal thin film because of compactness of a roll of tape. The preferred metal thin film for use as the magnetic layer of such media is an evaporated film of cobalt or cobalt-nickel alloy which is most often formed by oblique evaporation process wherein vacuum deposition of metal is conducted on a substrate at a varying incident angle with respect to a normal to the substrate.
Research is concentrated on media having a thinner ferromagnetic metal film for the purposes of compactness and extended recording time. Such media suffer from runnability, durability, physical strength of metal thin film, and some other properties. Such problems may be somewhat overcome by forming a metal thin film reinforcing layer on the rear surface of the substrate as disclosed in Japanese Patent Application Kokai Nos. 56-16939, 57-37737, 57-78627, and 58-97131, or by distributing fine particulates on the substrate surface to improve head contact and running performance as disclosed in Japanese Patent Application Kokai Nos. 58-68227 and 58-100221.
For the purpose of improving durability and electromagnetic properties, it is proposed to form a ferromagnetic metal thin film as a multi-layered structure of two or more layers. Reference is made to Japanese Patent Application Kokai Nos. 54-141608 and 57-130228, and Japanese Patent Publication No. 56-26892.
In general, magnetic recording media having a magnetic layer in the form of a ferromagnetic metal thin film is advantageous over the conventional coating type magnetic recording media because the former can be made thinner due to the absence of a binder and exhibits a higher saturation magnetization. The magnetic layers of binder-free magnetic recording media are formed by electro-plating, electroless plating, sputtering, vacuum deposition, ion plating or another metallizing technique. Absent a binder, metallized magnetic layers tend to be abraded away or broken during movement in frictional contact with the magnetic head, that is, by friction due to high speed relative movement with the magnetic head during operations of recording, reproducing and erasing magnetic signals. The metallized magnetic layers are liable to corrosion on the surface. A progress of corrosion detracts from such important properties as head contact and abrasion resistance and adversely affects electromagnetic properties.
One approach for reducing impact and friction to magnetic recording media is the application of lubricant on the surface of the magnetic layer as disclosed in Japanese Patent Publication No. 39-25246. For continuously supplying lubricant to the magnetic layer surface, it was proposed in Japanese Patent Publication No. 57-29769 to form a lubricating backcoat layer comprising a liquid or semi-solid lubricant and an organic binder on the surface of a magnetic recording medium opposite to the magnetic layer. When the magnetic recording medium is wound in a roll form, the lubricant blooming out of the backcoat layer migrates to the surface of the adjoining turns of the magnetic layer. The lubricant is thus continuously supplied to the surface of the magnetic layer which is increased in durability as demonstrated by mar and peel and can readily accommodate with a change of coefficient of dynamic friction.
The approach of Japanese Patent Publication No. 39-25246 suffers from the disadvantage that lubrication does not last and improvements in rust prevention and durability are not expectable because the lubricant is taken away by the head and other guide members. The provision of a lubricated backcoat by Japanese Patent Publication No. 57-29769 without forming a topcoat on the magnetic layer is still insufficient to reduce the friction between the magnetic layer and the head, incurring movement errors. When the lubricant is transferred from the backcoat layer to the magnetic layer uncovered with a topcoat, --although evaporated films having no oxygen incorporated therein (oxygen-free metallized films as disclosed in Japanese Patent Publication No. 57-29769) are not significantly affected--, commonly used evaporated films having oxygen incorporated therein (oxygen-containing metallized films) become unstable, incurring many problems including an output drop, head clogging, unfeasible image reproduction, relatively high frictional resistance, wear-out or even rupture of the films.
Like the approach of incorporating lubricant in the backcoat, it was also proposed to coat lubricant to a topcoat. The lubricant coating can reduce friction, but only for a limited period and it is also insufficient to improve rust prevention, corrosion resistance, and durability.
One effective topcoat applied to the magnetic metal thin film formed by the oblique evaporation process is a plasma-polymerized film of hydrocarbon as disclosed in Japanese Patent Application Kokai No. 59-72653, 59-154641, and 59-160828. Plasma-polymerized films formed by prior art methods, however, are still insufficent in corrosion resistance, runnability, and strength.
None of the prior art techniques have been successful in improving runnability, durability and ferromagnetic metal thin film strength while maintaining electromagnetic properties.